Circuits that convert a digital value, such as a sequence of binary digits, to an analog value, such as voltage or current, are known generally as digital to analog converters, or DACs. The output of a DAC can be an analog voltage or an analog current. In a typical DAC, a binary value is written into a register and circuitry converts this value into a voltage or current, which is available as an output from the DAC. It is often desirable that the analog output vary linearly with the digital input, and in this case, the conversion of the binary value into the voltage or current is a function of the gain and offset of the DAC. The voltage output of a linear DAC can be expressed as:Vout=(Vgain·(DATA/MAX))+Voffsetand the current output of a linear DAC can be expressed as:Iout=(Igain·(DATA/MAX))+Ioffsetwhere Vgain is the voltage gain, Voffset is the voltage offset, Igain is the current gain, Ioffset is the current offset, DATA is the digital input value and MAX is two raised to the number of bits in the digital input (e.g., 8192 in the case of a 13-bit digital input value). Voltage and current offsets and gains in general are not limited to positive values and can vary arbitrarily.
An existing method for setting the gain and offset of a DAC is to attach external precision resistors to the integrated circuit containing the DAC circuitry. The resistance of the resistors determines the gain and offset. In cases where multiple DACs are incorporated into a single integrated circuit, often the gain and offset for an entire group of DACs is set with a single pair of external resistors. A group of DACs can be of any size, and there can be any number of DACs implemented on a single device. The Edge6420, manufactured by Semtech Corporation, is an example of a device incorporating multiple groups of DACs. The Edge6420 has one group of 20 DACs, four groups of 8 DACs each, and one group of 12 DACs, for a total of 64 DACs. Typically DACs are configured either as voltage output DACs or current output DACs. In the case of the Edge6420, four of the groups consist of voltage output DACs and two of the groups consist of current output DACs.
The use of DACs dedicated to either voltage outputs or current outputs has several disadvantages. First, it decreases the flexibility of the device since the integrated circuit manufacturer fixes the number of current and voltage outputs. Alternatively, if user configuration of either current or voltage mode is desired, it increases the cost and power consumption of the device since two DACs would then be needed for a single output.
The use of external resistors to set the gain and offset for a group of DACs has several disadvantages. First, the customer must change the external resistors to change the gain or offset of a DAC group. This means that either a physical change to the device is needed, or the gain and offset must be fixed and cannot change dynamically. Also, since DACs may have a fairly large range of gains and offsets, a fairly large range of resistances is needed. Additionally, the precision of the gain and offset is limited by the precision of the resistors. Finally, the use of external resistors increases the cost of the end product (due to the cost of the resistors and the cost of additional board space), increases the cost of packaging due to additional pins, increases the cost of assembly, and increases the physical size of the end product.